[Version-unspecific] Log and drop GetObservationsStdTx unobservable ObservedTx

[Version-unspecific]

Intended to close #1709 (closed)
'GetObservationsStdTx errors on redundant ObservedTx and doesn't report details'.

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(In case of curiosity (regarding the second commit's rephrasing),
https://thornode-v1.ninerealms.com/txs/764c9cd00ab173d771674aec3a35a75ae599a6e9b733e9ec192d357224a7e25d
is an example of a Migrate transaction's ObservedTxIn and ObservedTxOut having different observed_pub_key contents
(in contrast with how a consolidate transaction would have the same one for both, since from the vault to itself).)